Interference often hinders performance of communication systems. One type of interference often encountered by a user within a communication system is interference generated by the transmissions of other users. This is typically caused by many users transmitting within the same channel (e.g., frequency band, time slot, or spreading code), and is referred to as co-channel interference. In order to reduce co-channel interference many communication systems employ a channel reuse pattern, where geographically adjacent transmitters transmit on different channels. For example, a cellular radio system defines geographic cells and assigns different channel sets to adjacent cells. Another common means of reducing co-channel interference is to employ smart dynamic resource management wherein users in close proximity are scheduled to use channels in a manner that creates minimum co-channel interference. These techniques can be employed only in systems where all channels are managed by a central resource manager that has knowledge of all resources and of all user communications needs. Centralized resource management requires that communication links be established between different users so that the channel allocation process can be controlled and monitored. Moreover, as systems evolve from circuit-centric operation to packet-centric operation, the channel utilization pattern becomes much more dynamic, leading to further implementation difficulties (e.g., the channel utilization may change more quickly than the response time of the channel allocation process, making it essentially ineffective). Future communications systems may not have a central system resource manager nor be able to define geographic reuse regions and, as a result, resource management may have to be distributed to all users.
Notwithstanding the above, more and more system operators are taking advantage of unlicensed frequency bands for transmitting information. Because the number of transmitters within an unlicensed frequency band is not restricted, there exists the potential of greatly increased co-channel interference. The co-channel interference problem in unlicensed frequency bands is exacerbated by the fact that it is desirable to minimally constrain users of these bands in order to encourage innovation. Thus, the usage rules for unlicensed frequency bands are typically defined in a simple etiquette. Operators within the unlicensed band typically do not have to synchronize to a common source, and co-channel interference is therefore often asynchronous in that the interfering signal does not align in time with the desired signal. Moreover, the etiquette may not require all users to use the same data modulation so that different users may not be able to decode one another's transmissions to obtain information that might be useful for mitigating co-channel interference.
The well known problem of hidden nodes further complicates spectrum sharing especially among heterogeneous users. The hidden node problem occurs when only one of the two transceivers of an active communications link can be detected by a third user that is exploring the possibility of sharing a specific channel. If the third user cannot detect the active user in the ongoing communications, the third user may infer that the channel is vacant and begin using that channel.
Because of the above-mentioned problems, a need exists for a method and apparatus for reducing both synchronous and asynchronous co-channel interference within an unlicensed communication system.